1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging lens having a three-lens structure that is capable of size and weight reduction. The imaging lens is used in an image-taking device that uses an image sensor element, such as a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras that utilize an image sensor element, such as the CCD, the CMOS, or the like, that is mounted on a portable computer, a television phone, a portable phone, a digital camera, and the like. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used in such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens and a two-lens structure lens system having two lenses have been used as such an imaging lens.
However, although such imaging lenses are extremely effective in reducing the size and weight of the lens system, the imaging lens cannot sufficiently handle high image quality and high resolution required of the imaging lenses in recent years.
Therefore, conventionally, a three-lens structure lens system using three lenses is used to handle high image quality and high resolution.
As examples of such a three-lens structure lens system attempting to enhance image quality and resolution, for example, the lens systems described in the following Patent Literatures 1 to 4 are known.    [Patent Literature 1] Japanese Patent Unexamined Publication 2005-173298    [Patent Literature 2] Japanese Patent Unexamined Publication 2005-91513    [Patent Literature 3] Japanese Patent Unexamined Publication 2003-322792    [Patent Literature 4] Japanese Patent Unexamined Publication 2004-163786
However, in the lens system described in Patent Literature 1, a third lens is a negative lens of which the convex surface faces the object side. Therefore, a balance between aberration correction and size reduction of the lens system is difficult to achieve. In addition, in power distribution of first to third lenses in the lens system described in Patent Literature 1 and a comparison of the center radius curvatures of both surfaces of the first lens, the back focus distance becomes too long. As a result, the overall length of the lens system becomes too long.
As in the lens system described in Patent Literature 1, in the power distribution of first to third lenses in the lens system described in Patent Literature 2, the back focus distance becomes too long. As a result, the overall length of the lens system becomes too long.
Furthermore, in the lens system described in Patent Literature 3, a third lens is a negative lens of which the convex surface faces the object side. Therefore, as in the lens system described in Patent Literature 1, a balance between aberration correction and size reduction of the lens system is difficult to achieve. In addition, in power distribution of first to third lenses in the lens system described in Patent Literature 3, the overall length of the optical system becomes too long. Moreover, in the lens system described in Patent Literature 3, the back focus distance is short and the third lens is a meniscus lens. Therefore, on an optical surface edge, a last surface is shaped to largely protrude toward an image-taking surface side, interfering with insertion of various filters.
Still further, in the power distribution of a first lens, a second lens, and a third lens in the lens system described in Patent Literature 4, the overall length of the lens system becomes too long.
Therefore, the conventional lens system was insufficient for actualizing further size reduction (such as shortening of the overall length) of the lens system itself, while maintaining excellent optical performance, such as high image quality and high resolution.